This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The primary aims of this study are to: 1) determine whether Mtor Inhibitor Sirolimus, administered orally twice daily on a continuous dosing schedule using pharmacokinetically-guided dosing: a) increases time to diseas progresssion based on volumetric MRI measurements in chldren and young adults with neurofibromatosis Type 1 (NF1) and inoperable progresssive plexiform neurofibromas (PN), b) results in objective radiographic responses based on volumetric MRI measurements in children and adults with NF1 and inoperable PN in the absence of documented radiographic progression at trial entry;2) evaluate the feasibility and toxicity of chronic Sirolimus adminstration in this patient population;and, 3) characterize the pharmacokinetic profile of Sirolimus when administered to this patient population.